The central region of the Milky Way is a dense and busy place. It is home to a supermassive black hole (Sagittarius A*) and a huge concentration of stars known as the Nuclear Star Cluster. In just 26 light-years, there are roughly 20 million stars. Now new research reported in three studies (here, here, and here) suggest that a small fraction of these stars did not form there.
An estimated 7 percent of the stars in the Nuclear Star Cluster have distinct properties compared to the remaining stellar bodies. In particular, this population has a different chemical composition compared to other stars: they are poorer in elements heavier than both helium and hydrogen, they move faster than the other stars, and the motion of their orbit appears tilted with respect to the disk of the Milky Way.
These clues led astronomers to put forward the hypothesis the stars formed together away from the core of the Milky Way. Yet questions remained: where did they form and when did they join the previous stars? When the team ran sophisticated computer simulations, they found these stars likely came to be in the Nuclear Star Cluster within the last 5 billion years.
For their birthplace, the team investigated the two most likely places. The stars may have once been the core of a dwarf galaxy orbiting the Milky Way or one of the many globular clusters of stars that exist around our galaxy. The most probable scenario sees a globular cluster forming between 10,000 and 16,000 light-years from the center of the Milky Way. This new population’s properties match well with what we see in other globular clusters around the Milky Way.
“Our results indicate that an infall of a rather nearby stellar cluster from the Milky Way itself is more likely,” co-author Dr Nadie Neumayer, from the Max Planck Institute For Astronomy, said in a statement.
“Although an extragalactic origin of the stars cannot be completely ruled out, it is rather unlikely,” added Dr Manuel Arca Sedda, ESO astronomer and lead author of one of the papers. “This is an additional sign that the central nuclear star cluster in the galaxy is at least partly the result of the impact of smaller clusters.”